This invention relates to equalizers for data communications systems and, more particularly, to adaptive equalizers for analog systems which rely on switched transmission media.
Data communications systems often employ transmission media having undesirable frequency dependant envelope delay (sometimes referred to herein as "phase") and attenuation distortion characteristics. Typically, the amount of transmission distortion the data suffers in such a system increases as a function of frequency. Consequently, as the data transmission rate is increased, successful data recovery ordinarily becomes increasingly dependent on there being adequate provision to compensate for or otherwise overcome such distortion.
Equalizers are commonly included in the receiving terminals of analog data communications systems to compensate for the transmission distortion. As extra dimension is, however, added to the problem of selecting an appropriate equalizer for a system which relies on a switched transmission medium, such as the public switched telephone network, inasmuch as it is generally impossible to accurately predict the level and frequency characteristics of the phase and attenuation distortion which will be encountered. Instead, the usual starting point in selecting an equalizer for such a system is to assume that the transmission distortion will lie somewhere within a predetermined range.
With that limitation in mind, it will be understood that there is a potentially serious data recovery problem in modern, high speed, analog facsimile systems and the like. As is known, the public switched telephone network is a favored transmission medium for facsimile communications. However, the reduced document transmission times of the newer analog facsimile systems, such as those which are based on the Telecopier 200 transceiver manufactured and sold by Xerox Corporation, are capable of being realized at acceptably low data error rates only if there is reasonably close compensation for the tansmission distortion the data suffers. As will be appreciated, the Xerox Telecopier 200 transceiver is a good example of the problem because of its document transmission times of approximately 2 and 3 minutes for standard 8 1/2 .times. 11 inches originals.
Others have previously recognized that the foregoing problem can be solved by using a so-called "adaptive equalizer" which is automatically adjusted to more or less complement the transmission distortion the data suffers. Indeed, substantial effort and expense have been devoted to the development of equalizers of that type, and some of the proposals have shown substantial promise, including the one described and claimed in a commonly assigned Torpie et al U.S. Pat. No. 3,798,547, which issued on Mar. 19, 1974, for an "Automatic Equalization Method and Apparatus." Nevertheless, there still is a need for a relatively economical and reliable adaptive equalizer which is substantially immune to the potentially disturbing effects of ordinary line noise.